This invention relates to the art of power dividing, or splitting, and combining networks for splitting or combining high frequency signals.
Although the invention will be described herein with particular reference as an RF splitter for use in conjunction with a modulator circuit within an RF transmitter, it is to be appreciated that the invention has broader applications and may be used in other areas such as in high fidelity audio and public address systems as either a high frequency splitter or a combiner.
In most prior art RF splitters, the phase and amplitude of the output signals vary substantially with variations in load impedance. For example, if an RF splitter be used for driving a plurality of MOSFET transistor amplifiers in a medium wave AM broadcast transmitter, the input impedance of one amplifier with its associated circuitry can differ from another amplifier by about 20%. With most RF splitters, if the impedance changes on the order of 20%, the voltage changes on the order of about 20%. It has been found that in order to obtain good efficiency, the voltage balance should be within 5% for class D MOSFET amplifiers.
It has been known in the prior art that a simple RF splitter having a small number of outputs, such as four, may well provide output signals having essentially equal amplitude and phase signals. However, if the number of outputs becomes large, as in a high power solid state transmitter operating on the order of 1 kilowatt, the outputs cannot physically tie to a point because the point becomes a stack or an area. The impedance of the drive circuit at some frequencies may be very low. The input impedance of switching type MOSFET transistors varies a large amount across the broadcast band. Unless all of the outputs originate from a point or a region that acts essentially as a point, the signals will not be equal with unequal loads.
It is therefore desirable that an RF splitter for such an application drive a plurality of N outputs from what appears to act as if each output came from essentially a point source.
Also, individual transformers may be constructed which are approximately ideal and with each having a fairly high transformation. In this case, each output may act as if it comes from a point source. However, constructing N transformers wherein each has a high turns ratio would be quite difficult and expensive.
It is therefore desirable that an RF splitter function similar to N ideal transformers but employing a transformer construction wherein all the windings are on a single core. In such case, the secondary windings may all be in parallel. In an actual implementation, the primary winding may be uniformly wound around a ferrite core to maintain a constant flux density around the core. Consequently, the voltage induced in the secondary is constant around its diameter.
Prior art examples of similar constructions include the U.S. Pat. No. 4,190,816 to Moss. This is a splitter which divides a high frequency signal into N output signals of the same power and phase. However, the output ports are not electrically connected in common. No disclosure is presented as to how each of the output ports may be used to provide a plurality of outputs.
The U.S. Pat. No. 2,738,464 to Abbett, is another teaching of employing transformer couplings, in this case as an RF voltage divider circuit. The couplings are tightly wound transformer elements. However, there is no teaching of a system wherein all of the outputs are commonly connected.